In the description of the background of the present invention that follows reference is made to certain structures and methods, however, such references should not necessarily be construed as an admission that these structures and methods qualify as prior art under the applicable statutory provisions. Applicants reserve the right to demonstrate that any of the referenced subject matter does not constitute prior art with regard to the present invention.
Cemented carbide cutting tools coated with various types of Al2O3 layers using Chemical Vapour Deposition (CVD), e.g., pure κ-Al2O3, mixtures of κ- and α-Al2O3 coarse grained α-Al2O3 and fine grained textured α-Al2O3 have been commercially available for years generally in multilayer combinations with other metal carbide and/or nitride layers, the metal being selected from transition metals of the IVB, VB and VIB groups of the Periodic Table.
Al2O3 crystallises in several different phases: α, κ, γ, δ, θ etc. The two most frequently occurring phases of CVD-produced wear resistant layers are the thermodynamically stable, α-phase and the metastable κ-phase or a mixture thereof. Generally, the κ-phase exhibits a grain size in the range 0.5-3.0 μm (dependent on the layer thickness) and the grains predominately grow through the whole coating forming a columnar type coating morphology.
Furthermore, the κ-Al2O3 layers are free from crystallographic defects and also free from micropores and voids.
Coarse grained (3-6 μm) α-Al2O3 often possesses porosity and crystallographic defects, while fine grained textured α-Al2O3 is free of defects with very pronounced columnar-shaped grains.
In U.S. Pat. No. 5,674,564 a method is disclosed of growing a fine grained κ-Al2O3 layer by employing a low deposition temperature and a high concentration of a sulphur compound.
In U.S. Pat. No. 5,487,625 a method is disclosed for obtaining a fine grained, (012)-textured α-Al2O3 layer consisting of columnar grains with a small cross section (about 1 μm).
In U.S. Pat. No. 5,766,782 a method is disclosed for obtaining a columnar fine grained (104)-textured α-Al2O3 layer.
The lifetime and the performance of a coated cutting tool are closely related to the microstructure of the coating materials used. Although, coatings produced according to above-mentioned prior art patents show good cutting properties there is still a strong desire to further improve coating microstructures to suit specific cutting conditions and workpiece materials.
As has been mentioned above, all Al2O3 layers produced by the CVD technique possess a more or less columnarlike grain structure. An Al2O3 layer with an equiaxed grain structure is, however, expected to show some favourable mechanical properties, e.g., resistance to crack propagation and higher cutting edge toughness, as compared to a layer with a columnar grain structure. In addition, fine grained layers generally have smoother surfaces than coarse grained layers. During cutting less workpiece materials will adhere onto a smooth coating surface which in turn will imply lower cutting forces and less tendency for the coating to flake off. Nowadays coated cutting inserts are often brushed with SiC based brushes or blasted with fine grained Al2O3 powder in order to obtain a smooth coating surfaces, a rather costly production step.
One well-known and possible technique to produce a fine grained structure and to restrain a columnar grain growth is to deposit a so-called multilayer structure in which the columnar growth of, e.g., Al2O3, is periodically interrupted by the growth of a 0.05-1 μm thick layer of a different material such as disclosed in U.S. Pat. No. 4,984,940 and U.S. Pat. No. 5,700,569. The latter layer should preferably have a different crystal structure or at least different lattice spacings in order to be able to initiate renucleation of the original layer. One example of such a technique is when the Al2O3 growth periodically is interrupted by a short TiN deposition process resulting in a (Al2O3+TiN)×n multilayer structure with a thickness of each individual TiN layer in the range 0.1-1 μm, e.g., see Proceedings of the 12th European CVD Conference page pr. 8-349. However such multilayer structures very often suffer from a low adherence between the two different types of layers.
In Swedish patent application SE 0004272-1 a method is disclosed for obtaining a fine grained α-Al2O3 layer consisting of essentially equiaxed grains with a grain size <1 μm. The grain refinement is accomplished by periodically interrupting the Al2O3 process and treating the Al2O3 surface with a mixture of TiCl4/H2. When the Al2O3 process is restarted renucleation of the α-Al2O3 will take place.
It is not possible to use this method to produce fine grained κ-Al2O3 since only the α-Al2O3 phase will nucleate on the TiCl4/H2 treated Al2O3 surface.
κ-Al2O3 and α-Al2O3 coatings used as tool materials have slightly different wear properties when cutting different materials. It is therefore also desirable to have means to produce fine grained κ-Al2O3 with a controllable grain structure.